Atoms form molecules by means of their electrons. Electrons move very fast through the atoms in a way which makes it impossible to determine where an electron is located at a given time. There is only a statistical certainty that electrons are in a given zone around the nucleus of the atom. Which zone this is depends on the amount of energy that an atom has at a certain moment in time.
The different zones in an atom could be somewhat compared to an onion, whereby every layer could be a limited zone where electrons can be found. These layers are called “shells”. In the inner shell, closest to the nucleus of the atom there is only space available for 2 electrons. For H (hydrogen) with 1 electron and He (helium) with two electrons this is no problem. There is enough space available. But for Li (lithium) with 3 electrons another solution is necessary. So the third electron moves to the following shell which is situated more outwards from the nucleus. This 2nd shell can contain 8 electrons, just as the 3rd shell. The 4th and the 5th shell can contain up to 18 electrons, the 6th shell up to 32 electrons.
Photo: Lithium atom - author: Pumbaa
Every time a shell has been filled the other electrons are moving up to the next shell. Chlorine with 17 electrons has 2 electrons in the 1st shell, 8 in the 2nd shell and 7 in the 3rd shell. The more shells there are, the larger the atom. We can see this by comparing the diameters of C (6 electrons) and gold (79 electrons). The diameter of C is only about half the size of the diameter of gold.
Atoms continuously strive to reach full electron shells. They do this by taking up, giving off or sharing electrons with other atoms. It is a mechanism depending on chemical reactions and leading to the formation of molecules.
Covalent bonds
Let us look at the molecule “methane”. Methane consists of 1 carbon atom (C) and 4 hydrogen atoms (H). Carbon has 6 electrons, of which 2 are on the inner shell and 4 free electrons on its outer shell (2nd shell with ultimately 8 electrons). Carbon is 4 electrons short of a full outer shell. Hydrogen lacks one electron in its outer shell (1st shell with maximum 2 electrons). The carbon atom needs to combine with 4 hydrogen atoms to fill up its outer shell (4 + 4x1 electrons). Hydrogen only needs 1 electron to complete its outer shell. So the carbon atom unites with 4 hydrogen atoms, forming 4 electron pairs (4 x 2), resulting in full outer shells for all atoms. By doing this methane becomes a stable molecule.
Photo: Methane molecule - author not found
This kind of binding is called a “covalent bond”. The electrons are shared, on an equal basis, between the carbon atom and the 4 hydrogen atoms. That way each atom has, notwithstanding the motions of the electrons, always a full outer shell. The molecule has no extra free spaces anymore and therefore is in equilibrium. It has no need anymore to form new relations. It forms therefore a stable unit. Only when heavy collisions would occur or when enough external energy would be added, bonds could be broken.
The result is a chemical reaction. This is what happens when we burn methane as natural gas for cooking or heating. Methane molecules have no common interest in each other. They live in a loose connection that we call a “gaseous form”.
Photo:Combustion of methane - author: Essent Energy
When we look at a water molecule we see a different story. Water exists of 1 oxygen atom with 8 electrons, (of which 2 are in the inner shell and 6 in the outer shell) and 2 hydrogen atoms (each with 1 electron on the outer shell). Oxygen needs 2 electrons to complete its outer shell. But oxygen is quite dominant and does not share the electrons on an equal basis. The oxygen atom sits between the two hydrogen atoms and in fact attracts the hydrogen electrons towards the center with as result that these electrons are closer to the oxygen nucleus than they would be in methane.
The result is that the hydrogen atom, because it partly looses a negative ion, receives a small positive charge. The opposite is true for oxygen. Because this atom attracts 2 negative electrons, it gets a more negative charge. We know that equal charges reject each other and that opposite charges attract each other. Because there are two slightly positive atoms in water these move slightly away from each other. You can see this in the drawing below. We call this effect “polarity” and we call this kind of molecule “polar”. Polarity has an effect on the properties of the molecule.
Photo: Polarity of water - author: Dan Craggs
The reason why water molecules attract each other is due to the fact that the negative oxygen atom of one molecule attracts the positive hydrogen atoms of another water molecule. This way mutual connections are formed which we call “hydrogen bonds”. These bonds are partly responsible for the inner structure of cellulose and responsible for binding water in, for example, paper.
Photo: hydrogen bonds - author: Qwerter
Covalent bonds are mostly fairly weak bonds. Most molecules with covalent bonds have a liquid or gaseous form. The compounds they form mostly have relatively low melt- and cooking points.
Guy De Witte
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